The present invention relates to a method for obtaining pigments for ceramic and glass, particularly for substrates and coatings, and to the pigments obtained thereby.
In the field of production of ceramic and glass articles, such as for example tiles, kitchenware, sanitary fittings, furnishings and ornaments, gift items and the like, one of the steps of greatest technical and commercial interest is the coloring of the substrates, i.e., of the bodies of the articles, and of their coatings and decorations.
The search for new or alternative pigments and the development of existing ones is evolving constantly.
The essential requirements for a product to be usable as a ceramic/glass pigment are: thermal stability, i.e., resistance to the firing cycles to which the articles are subjected and which can reach temperatures on the order of 1400° C.; chemical resistance, i.e., resistance to the chemical attack of some substances that are part of the composition of the body or glassy ceramic cladding, such as for example silicates; availability; constant quality, particularly in terms of tinting strength and tint; and low cost.
Synthetic pigments are known which are obtained from the high-temperature reaction of various chemical products, which by mutually combining form a product that satisfies the necessary requirements.
Pigments constituted by natural minerals are also known, but they have the drawback of a variable and inconstant behavior, particularly in terms of thermal stability and constant quality (tinting strength and tint).
These last pigments include, for example, a natural mineral known as gres de Thiviers, which originates from a sedimentary rock that is present in particular in France near Limoges.
Gres de Thiviers is a sand-like detritus that contains iron in the form of partially hydrated oxide together with limited quantities of other minerals, including silicoaluminates, carbonates, phosphates and others.
Gres de Thiviers is extracted and subjected to a treatment during which the siliceous fraction, whose structure probably contains the iron oxide accumulated by precipitation during the formation of the sediment, is enriched.
The chemical analysis of gres de Thiviers is highly variable: on average, it contains SiO2 in amounts variable from 78 to 86%, iron expressed as Fe2O3 in amounts variable from 11 to 15%, and impurities such as CaO, Al2O3, P2O5, and others.
In its natural state, gres de Thiviers has a more or less intense ochre yellow color, which bears witness to the fact that the iron oxide is present, at least to a large extent, in hydrated form, and assumes a coral pink color if it is subjected to a thermal cycle for firing at more than 850° C.
The use of gres de Thiviers as a pigment is known and widespread owing to the particular color tint of pink that it gives to ceramic and glass bodies and coatings; however, its use is limited by the inconstancy of its composition, by the variability of its behavior, particularly in terms of tint and coloring intensity, and by its very high cost.
Many attempts have been made to synthesize colors similar to gres de Thiviers, but all have had poor results; the synthesized pigments in fact produce a coloring that is closer to wine pink and brown than to coral pink, especially due to a significant quantity of iron oxide that is not inserted in the crystalline lattice of the silica and is still present as such in their structure.